An experimental investigation of ethylene/O2/diluent mixtures: Laminar flame speeds with preheat and ignition delays at high pressures

The atmospheric pressure laminar flame speeds of premixed ethylene/O₂/N₂ mixtures were experimentally measured over equivalence ratios ranging from 0.5 to 1.4 and mixture preheat temperatures varying from 298 to 470 K in a counterflow configuration. Ignition delay measurements were also conducted for ethylene/O₂/N₂/Ar mixtures using a rapid compression machine at compressed pressures from 15 to 50 bar and in the compressed temperature range from 850 to 1050 K. The experimental laminar flame speeds and ignition delays were then compared to the computed values using two existing chemical kinetic mechanisms. Results show that while the laminar flame speeds are reasonably predicted at room temperature conditions, the discrepancy becomes larger with increasing preheat temperature. A comparison of experimental and computational ignition delay times was also conducted and discussed. Sensitivity analysis further shows that the ignition delay is highly sensitive to the reactions of the vinyl radical with molecular oxygen. The reaction of ethylene with the HO₂ radical was also found to be important for autoignition under the current experimental conditions.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Ignition of CO/H2/N2 versus heated air in counterflow: experimental and modeling results

Nonpremixed ignition in counterflowing CO/H₂ vs. heated air jets is experimentally and computationally investigated. The experiments confirm the numerical modeling observation of the existence of three ignition regimes as a function of the hydrogen concentration. In all three regimes, we first detect experimentally the onset of chemiluminescent glow due to excited CO₂ followed by flame ignition, as the temperature of the air jet is raised gradually. The temperature extent of the glow regime, however, is progressively reduced with increasing hydrogen addition; no glow is detected for H₂ concentrations in excess of 73%. The temperatures for glow onset and flame ignition are represented by the boundary air temperatures for each threshold. The variation of these temperatures with system pressure and flow strain rate is explored, for pressures between 0.16 and 5 atm, and strain rates of 100 to 600 s⁻¹. The pressure variation is found to result in three p-T ignition limits, similar to the ignition limits observed in the H₂/O₂ system. This similarity is also observed on the effects of aerodynamic transport on ignition: within the second limit the ignition temperatures are found to be essentially insensitive to flow strain rate, whereas the other two limits are significantly affected by strain. The transport insensitivity is maintained even in the limit of very low H₂ concentrations, where an analogous H₂/N₂ mixture would fail to ignite. This behavior is explained computationally by the replacement of the shift reaction OH + H₂ → H₂O + H with the reaction CO + OH → CO₂ + H, thereby minimizing the effect of diminishing H₂ concentration. The experimental data are found to agree well with the calculated results, although discrepancies are noted in modeling the onset of chemiluminescence and its response to pressure variations.     

Comparison of species profiles between O2 and NO2 oxidizers in premixed methane flames

The profiles of the species H, OH, CH, NH, CN, NCO, NO₂, and CH₃O are compared in a series of five premixed stoichiometric 15-torr CH₄/O₂/NO₂/N₂ flames with NO₂ comprising between 0% and 40% of the oxidizer. Relative species concentrations were measured by laser-induced fluorescence (LIF) and these results are compared with calculations using measured temperature profiles. The reaction mechanism of Miller and Bowman incorrectly predicts the standoff from the burner in flames containing more than 20% NO₂; addition of several reactions involving NO₂ and HONO produces excellent agreement with experiment for most species. The reaction CH₃ + NO₂ → CH₃O + NO is found to be particularly important in the reaction mechanism. LIF profiles of CH₃O show this species to be present in far larger quantities in the NO₂ supported flames than in the CH₄/O₂ system. The nitrogen-containing intermediates CN, NCO, and NH are all overpredicted by a factor of two in the 40% NO₂ flame relative to the 10% NO₂ flame. This indicates an inaccuracy in either the reburn reactions or the fuel nitrogen chemistry when large amounts of NO are present. The kinetic modeling shows that in the 40% NO₂ flame, the dominant pathway to N₂ formation is through N₂O, which is produced primarily by the reaction of NCO with NO. Comparison of emission profiles of NO₂* for the various flames indicates that the appearance of an orange-yellow luminous zone at the base of NO₂ supported flames is caused by thermal excitation of NO₂, not by a chemiluminescence mechanism.     

Measurement of propagation speeds in adiabatic flat and cellular premixed flames of C2H6+O2+CO2

Adiabatic burning velocities of premixed flat flames and propagation speeds of adiabatic cellular flames of mixtures of ethane+oxygen+carbon dioxide are reported. The oxygen content O₂/(O₂+CO₂) in the artificial air was varied from 26 to 35%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. A heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. Measurements in cellular flames are presented and compared with those for laminar flat flames and also with qualitative predictions of a theoretical model. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Cellularity disappears when the flames become only slightly subadiabatic.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures

Experimental and numerical study on laminar burning velocity of H₂/CO/N₂/CO₂/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4–1.0). This is mainly due to the high reactivity of H₂ leading to high production rate of H and OH radicals. Reactions  and  play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and reaction R31 plays the dominant role for mixtures with low hydrogen fraction.     

Experiments on the scalar structure of turbulent CO/H2/N2 jet flames

Scalar and velocity measurements are reported for two turbulent jet flames of CO/H₂/N₂ (40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters (4.58 mm and 7.72 mm). Simultaneous measurements of temperature, the major species, OH, and NO are obtained using the combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence. Three-component laser-Doppler velocimetry measurements on the same flames were performed at ETH Zurich and are reported separately. This paper focuses on the scalar results but includes some limited velocity data. Axial profiles of mixture fraction, major species mole fractions, and velocity in these two flames are in close agreement when streamwise distance is scaled by nozzle diameter. However, OH mole fractions are lower and NO mole fractions are higher near the stoichiometric flame length in the larger flame due to the lower scalar dissipation rates and longer residence times. Turbulent flame measurements are compared with steady strained laminar flame calculations. Laminar calculations show remarkably close agreement with measured conditional means of the major species when all diffusivities are set equal to the thermal diffusivity. In contrast, laminar flame calculations that include the normal Chemkin treatment of molecular transport are clearly inconsistent with the measurements. These results suggest that turbulent stirring has a greater influence than molecular diffusion in determining major species concentrations at the flow conditions and locations considered in the present experiments, which begin at an axial distance of 20 nozzle diameters. Analysis of the conditional statistics of the differential diffusion parameter supports this conclusion, though some evidence of differential diffusion is observed. With regard to validation of turbulent combustion models, this data set provides a target that retains the geometric simplicity of the unpiloted jet flame in coflow, while including a chemical kinetic system of intermediate complexity between hydrogen flames and the simplest hydrocarbon flames. Aspects of the measurements, including Favre-averaged profiles, conditional statistics, mixture fraction pdf’s, and departures from partial equilibrium, are presented and discussed in terms or their relevance to the testing of turbulent combustion submodels. The complete data are available on the World Wide Web for use in model validation studies.     

Onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a flat-flame heat-flux burner

The onset of cellular instability in adiabatic H₂/O₂/N₂ premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H₂/O₂/N₂ flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%–30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2–3 times of the burning velocity of the corresponding planar flame.     

Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

Experimental measurement of the laminar burning velocities of H₂/CO/air mixtures and equimolar H₂/CO mixtures diluted with N₂ and CO₂ up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N₂ and CO₂ dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO₂ due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H₂ has a destabilizing effect which is the most dominant. A higher N₂ and CO₂ dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.     

Numerical simulation of one-dimensional laminar flames propagating into reacting premixed gases

In this paper, the propagation of a one-dimensional flame front into a reacting combustible mixture was numerically studied. A simplified mathematical method, splitting the problem into a prereaction part and a flame propagation part, was applied to the completely nonstationary problem. Initially stoichiometric mixtures of H₂/O₂ and C₂H₆/O₂ were investigated for isothermal and adiabatic boundary conditions of the prereactions. In the isothermal case, the laminar burning velocity of the mixture decreased gradually with time. In conclusion, the reasons for this decrease are an increasing amount of combustion products combined with the heat loss necessary to maintain isothermal conditions. Compared with these phenomena, the accelerating effect of radical concentrations in the preflame region is of minor importance. In the adiabatic case, the laminar burning velocity increases steadily, until the ignition delay time of the initial mixture is reached. In this period, the accelerating effect resulting from the temperature increase in the preflame region dominates the decelerating effect of the increasing product concentration. The flame thickness, which was also computed for both boundary conditions, increases here for all examined flames during the time-dependent propagation through the reacting gas mixture. This change in thickness proceeds gradually in the isothermal and spontaneously in the adiabatic case.     

High temperature rate coefficient for the reaction of O(P) with H2 obtained by the resonance absorption of O and H atoms

The reaction of O(³P) with H₂ has been studied behind reflected shock waves in the temperature range of 1713–3532K at total pressures of about 1.4–2.0 bar by Atomic Resonance Absorption Spectroscopy using mixtures of N₂O and H₂ highly diluted in Ar. The O atoms were generated by the fast thermal decomposition of N₂O and the reaction with H₂ was followed by monitoring the time dependent O and H atom concentrations in the postshock reaction zone. For the experimental conditions chosen, the measured O and H atom concentrations were primarily sensitive to the well-known N₂O dissociation and to the studied reaction and hence its rate coefficient could be deduced. The measured rate coefficient data are fitted by the least-squares method to obtain the following three parameter expression: K₄=3.72×10⁶(T/K)^2.17exp(−4080K/T)cm³ mol⁻¹8⁻, which is in excellent agreement with the recent ab initio calculations for the rate coefficient of this reaction in the overlapping temperature range. The present result is also compared to the experimental results reported by earlier investigators.     

Comparisons of the structure of stoichiometric CH4---N2O---Ar and CH4---O2---Ar flames by molecular beam sampling and mass spectrometric analysis

Measurements are reported of the profiles of composition and temperature in laminar premixed flat flames of CH₄---N₂O---Ar and CH₄---O₂---Ar. Measurements were made in near stoichiometric mixtures at 30 torr by molecular beam sampling and mass spectrometric sample analysis. All major stable species and many important unstable species were measured by this technique, many species being identified in the flame with N₂O as oxidizer for the first time. Calibration of the concentration profiles was accomplished by the use of calibration gases for stable species and by comparison of the mass spectrometer signal in the well-characterized CH₄---O₂---Ar flame with signals in the CH₄---N₂O---Ar flame and by partial equilibrium for the hydrogen---oxygen system. The measurements have identified the presence of NCO, HCN, and HNCO as reaction intermediates and the importance of these species in the reaction mechanism is discussed.     

The effects of composition on burning velocity and nitric oxide formation in laminar premixed flames of CH4 + H2 + O2 + N2

Experimental measurements of adiabatic burning velocity and NO formation in (CH₄ + H₂) + (O₂ + N₂) flames are presented. The hydrogen content in the fuel was varied from 0 to 35% and the oxygen content in the air from 20.9 to 16%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + nitrogen + oxygen mixtures were found in satisfactory agreement with the modeling. The NO concentrations in these flames were measured in the burnt gases at a fixed distance from the burner using probe sampling. In lean flames, enrichment by hydrogen has little effect on [NO], while in rich flames, the concentration of nitric oxide decreases significantly. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions and trends were found in good agreement with the experiments. Different responses of stretched and nonstretched flames to enrichment by hydrogen are demonstrated and discussed.     

Laminar burning velocities and Markstein lengths of premixed methane/air flames near the lean flammability limit in microgravity

Effects of flame stretch on the laminar burning velocities of near-limit fuel-lean methane/air flames have been studied experimentally using a microgravity environment to minimize the complications of buoyancy. Outwardly propagating spherical flames were employed to assess the sensitivities of the laminar burning velocity to flame stretch, represented by Markstein lengths, and the fundamental laminar burning velocities of unstretched flames. Resulting data were reported for methane/air mixtures at ambient temperature and pressure, over the specific range of equivalence ratio that extended from 0.512 (the microgravity flammability limit found in the combustion chamber) to 0.601. Present measurements of unstretched laminar burning velocities were in good agreement with the unique existing microgravity data set at all measured equivalence ratios. Most of previous 1-g experiments using a variety of experimental techniques, however, appeared to give significantly higher burning velocities than the microgravity results. Furthermore, the burning velocities predicted by three chemical reaction mechanisms, which have been tuned primarily under off-limit conditions, were also considerably higher than the present experimental data. Additional results of the present investigation were derived for the overall activation energy and corresponding Zeldovich numbers, and the variation of the global flame Lewis numbers with equivalence ratio. The implications of these results were discussed.     

Laser-induced fluorescence measurements of NCN in low-pressure CH4/O2/N2 flames and its role in prompt NO formation

NCN profiles were measured for five rich and lean premixed, low-pressure methane flames using laser-induced fluorescence (LIF). A semiquantitative determination of the NCN mole fractions as a function of spatial height above the burner is made by calibrating the NCN LIF signals using highly accurate OH LIF measurements in an adjacent spectral region. The resulting calibration yields an uncertainty estimate of a factor of 3 for the absolute values, but only ±25% for the relative NCN profiles. For all flame conditions, the NCN profiles occur immediately downstream of previously measured CH profiles. In addition, high correlations are found between the peak CH and peak NCN concentrations and the peak NCN and postflame NO concentrations over all equivalence ratios. These observations are consistent with NCN being the primary product channel from the CH + N₂ reaction and the initial intermediate in the prompt NO formation. This is the first mechanistic study in hydrocarbon flames that provides such experimental evidence. The experimental profiles are compared to numerical calculations using modified versions of two well-established hydrocarbon kinetic mechanisms. Reasonable agreement between the calculations and experiment is found for NCN profile shape, location of peak NCN concentrations, and absolute mole fractions. However, the dependence on stoichiometry of the peak NCN concentration is overestimated. Further work is required on NCN kinetics for modeling prompt NO in laminar premixed flames.     

Energy-efficient conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 catalyst in the presence of limited oxygen

Oxidative conversion of propane to propylene and ethylene over a V₂O₅/CeO₂/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O₂, has been studied at different temperatures (700–850 °C), C₃H₈/O₂ ratio (4.0), H₂O/C₃H₈ ratio (0.5) and space velocity (3000 cm³ g⁻¹ h⁻¹). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O₂, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O₂. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process.     

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H₂/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H₂/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N₂ diluted synthetic H₂ and H₂/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.     

Inhibition of premixed carbon monoxide–hydrogen–oxygen–nitrogen flames by iron pentacarbonyl

This paper presents measurements of the burning velocity of premixed CO–H₂–O₂–N₂ flames with and without the inhibitor Fe(CO)₅ over a range of initial H₂ and O₂ mole fractions. A numerical model is used to simulate the flame inhibition using a gas-phase chemical mechanism. For the uninhibited flames, predictions of burning velocity are excellent and for the inhibited flames, the qualitative agreement is good. The agreement depends strongly on the rate of the CO + OH ↔ CO₂ + H reaction and the rates of several key iron reactions in catalytic H- and O-atom scavenging cycles. Most of the chemical inhibition occurs through a catalytic cycle that converts O atoms into O₂ molecules. This O-atom cycle is not important in methane flames. The H-atom cycle that causes most of the radical scavenging in the methane flames is also active in CO–H₂ flames, but is of secondary importance. To vary the role of the H- and O-atom radical pools, the experiments and calculations are performed over a range of oxygen and hydrogen mole fraction. The degree of inhibition is shown to be related to the fraction of the net H- and O-atom destruction through the iron species catalytic cycles. The O-atom cycle saturates at a relatively low inhibitor mole fraction (100 ppm), whereas the H-atom cycle saturates at a much higher inhibitor mole fraction (400 ppm). The calculations reinforce the previously suggested idea that catalytic cycle saturation effects may limit the achievable degree of chemical inhibition.